The present invention relates to refrigeration systems for refrigerating or air conditioning a compartment such as the passenger compartment of a vehicle. Typically, air conditioning systems for passenger vehicles employ an engine driven compressor for circulating a charge of highly compressed refrigerant gas through an exothermic heat exchanger or condenser cooled by ambient air and the cooled liquid refrigerant flows through an expansion device for creating a substantial pressure drop to enable the condensed refrigerant to be circulated through an endothermic heat exchanger or evaporator located in the compartment to be conditioned or refrigerated. Heat absorbed by the evaporator boils and vaporizes the refrigerant which is returned as a superheat gaseous form to the inlet of the compressor. It has been found necessary to maintain a slight degree of superheat, i.e. vaporized refrigerant slightly above its saturation temperature, in order to provide maximum efficiency of the heat exchange in the evaporator and also to prevent return of liquid to the compressor and consequent compressor damage.
Heretofore, the pressure drop in the refrigerant between the condenser and evaporator has been provided by either a capillary tube or an expansion control valve. Typically, vehicle air conditioning system expansion control valves are pressure operated temperature sensing devices which respond to the temperature of the refrigerant to expand fluid in a closed chamber to act upon a diaphragm and control the position of the flow valve member. Examples of such valves are those described in U.S. Pat. No. 3,667,247 and U.S. Pat. No. 3,810,366. Pressure operated refrigerant expansion valves respond to an existing temperature condition of the refrigerant and thus control of the refrigerant flow lags the temperature of the air to be conditioned in the compartment.
It has thus been proposed to provide an electrically actuated expansion valve for controlling the flow of refrigerant in the vehicle air conditioning system, in order that the valves may be made to respond to an electrical control signal which can be generated by a microcomputer to anticipate the condition of the air in the passenger compartment based upon multiple sensed inputs to the microcomputer. In order to provide a vehicle air conditioning system which has the refrigerant flow controlled by an electrically operated expansion valve, it has been found necessary to provide an indication of the condition of the vaporized refrigerant returning to the compressor from the evaporator.
In co-pending application Ser. No. 007,147, titled "Controlling Refrigeration" filed Jan. 27, 1987, and assigned to the assignee of the present invention, it is disclosed that an electrically operated expansion valve can be employed in an automotive air conditioning system and controlled by comparing temperatures sensed by thermistors disposed respectively to sense the refrigerant temperature at the inlet and outlet of the evaporator. The thermistors provide electrical temperature indications to a microcomputer which generates a width-modulated pulse control signal having the pulse width or duty cycle varied in accordance with the temperature comparison to provide continuous control of the refrigerant flow to the valve. The system disclosed in the aforesaid co-pending application functions to maintain the refrigerant flow at a uniform temperature at the inlet and the outlet of the evaporator thereby maintaining slightly superheated vaporized refrigerant at the outlet of the evaporator. However, this type of control system is usable only in refrigeration or air conditioning systems having a relatively low pressure drop across the evaporator on the order of 25 psi (1.75 kg/cm.sup.2).
Where it is desired to provide electrical control of a refrigerant expansion valve in an automotive air conditioning system, in applications where a relatively high pressure drop is permitted across the evaporator e.g. greater than 25 psi (1.75 kg/cm.sup.2), it has been found difficult to provide a way or means of economically providing necessary inputs to the microcomputer and further has been found difficult to generate a satisfactory control signal for the valve which could anticipate and respond to variations in system thermal loading in a manner to produce the desired regulation of the air temperature in the passenger compartment. It has, therefore, been desired to find an economical way or means of electrically controlling refrigerant flow in a vehicle air conditioning system by an electrically operated expansion valve and to provide a degree of control sophistication and response such that the passenger compartment can be maintained at a relative constant comfort level despite a wide variation in ambient conditions and thermal loading of the system.